Dual monitors

790 has intel 2nd gen sandy bridge cpu which has Intel® HD Graphics 2000. it supports dual display via rear VGA and Display port.

It does not however support using INTEL Graphics when a video card is installed in the X16 slot.

https://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/optiplex-790-tech-guide.pdf

2nd monitor will likely need display port to vga

https://www.amazon.com/dp/B017Q8OCSY

ports on back

If ANY card is installed in the X16 slot it will not post.

It will display

System halted!
Attention: Unsupported Video Configuration Detected

Action is Required

This computer has an add-in graphics card, but the monitor is plugged
into the integrated video connector.
To attach the monitor cable to the add-in graphics card:

1. Shut down the computer
2. Plug the monitor cable into the add-in graphics card connector.
This may require a video adapter or video adapter cable.
(provided with the system)
3. Turn on the computer.

This message should not appear after completing these steps.
For more information or help, please refer to the system documentation

a user validated two monitors on 790 SFF "The graphics card is intergrated with the motherboard"

https://www.adverts.ie/desktops/dell-optiplex-790-with-dual-21in-monitors-complete-with-stand-price-reduced/19411782